In 2006, she won a silver medal at the Scottish School's Championships in the 1500 metres and a bronze medal at the International Under-17s Schools Championships.[7] In 2007, she came fifth in the Scottish Senior Championships 1500 metres.[7] At the 2008 Scottish Indoor Championships she won a bronze medal in the 1500 metres.[7]

McColgan was selected for the 2008 Commonwealth Youth Games in India but suffered a major knee injury from which took her a year-and-a-half to recover.[6] She also missed the 2011 World Championships, held in Daegu, South Korea, after breaking her foot at a Diamond League event in London.[8] Her senior debut for Great Britain came at the 2011 European Team Championships in Stockholm, Sweden. Initially selected as a travelling reserve to cover a range of events, she was called upon to compete in the 3000 metres steeplechase, an event which she had only competed in twice before, and ran a personal best while finishing ninth.[9][10] She came fourth in the 1500 metres at the 2011 UK Indoor Championships and won the silver medal in the 5000 metres at the UK Outdoor Championships.[7] She won the gold medal in the 1500 metres at the 2011 Scottish University Championships.[7]

She won two gold medals at the 2012 Scottish Universities Indoor Championships, placing first in both the 1500 and 3000 metres events.[7] In June 2012 at a meeting in Oslo, Norway she ran a time of nine minutes 38.45 seconds to beat the Olympic 'A' qualification standard in the 3000 metres steeplechase.[11] McColgan confirmed her qualification for the steeplechase at the 2012 Summer Olympics in London by winning the British trials in a time of nine minutes 56.90 seconds.[12] She was the only athlete to finish in under ten minutes and the only British athlete to have achieved the 'A' qualifying standard.[13][14] After achieving Olympic qualification McColgan chose not to compete at the 2012 European Athletics Championships in Helsinki, Finland.[15]
She competed for Great Britain team for the 2012 Summer Olympics in the women's 3000 metres steeplechase alongside compatriot Barbara Parker.[16] Her time of 9:54.36 was not sufficient for her to reach the final.[17]